When an electrically powered vehicle is designed to receive its power from an external source through an overhead trolley wire instead of a third-rail, the power wire must be suspended in such a way that the underneath surface of the wire is not obstructed. This is necessary because the vehicle has an upwardly and rearwardly extending pole to make contact with the trolley wire, the pole having a harp and contact shoe assembly or the like at its upper end which rides along against the wire. If there are obstructions along the wire, the shoe is likely to be damaged or to jump off of the wire, opening the circuit and necessitating manual replacement of the shoe against the wire.
In one arrangement, a messenger wire is suspended between supports and follows a catenary curve. The trolley wire is then suspended below the messenger wire by a plurality of hangers which somehow engage both the messenger wire and the trolley wire. In known hangers for this purpose, the upper end of the hanger loops over the messenger wire and is provided with a releasable clip at its lower end. The trolley wire is grooved so that it can be engaged by the clip above a horizontal catenary surface passing through its central axis, thereby leaving the lower portion unobstructed as desired.
While these hangers are usable, they have some serious disadvantages. It is important for the hangers to be easily removable and replaceable for maintenance but, once attached, the hangers must not release by themselves. Hangers of the prior art are subject to loosening and release because of the vibration to which the trolley wire is subject and are therefore not sufficiently reliable. One such clip uses a threaded fastener to hold cast clip halves together but the threaded fastener rather easily vibrates apart, releasing the trolley wire. That same hanger also has the disadvantages of significant weight because of the cast clip, a plurality of parts, considerable difficulty of manufacture, and the need for a special tool to lock the clip.
Another prior art clip uses a rotatable locking device which passes through an irregular opening in the clip and is then rotated to lock the clip in its trolley wire-engaging position. However, the rotatable device is also subject to being loosened and turned by vibration and, in an apparent effort to make the structure more secure, the clip itself is made of a very heavy gauge material, adding unnecessary and undesirable weight. This also requires larger force on installation which, in turn, creates larger permanent deformation of the camming surfaces, thereby limiting the number of possible re-installations of the clip and shortening its useful life. The clip opening which receives the locking device is also a complex shape, increasing the cost of the tool which is used to form the clip and also increasing the tool maintenance costs, and the clip further requires additional openings for attachment to the rest of the hanger.